Why are they so heavy?

[jimjet]

I am fairly new to the jet ski world but I am a good friend of nickwads and have assisted him when laminating the big parts. Even tropical slow hardener goes off quickly here at 30 degrees c, so you need helpers to get the job done before the resin goes off.
I am curious though as to what the factory Jetskis are made out of and why so heavy?
I make fibreglass moulded and vacuumed model aircraft using epoxy and you get a very strong and lightweight structure using these materials.
Checking out a Hull that Nick has, (I won't say the make) it was 78 kilos bare and in places was about 10mm thick.
Is this necessary?
Does a heavy hull ride better?
Sorry just a novice wanting to learn.
Hoping to ride Nicks new one this weekend. I reckon you just need loads of beer and go for it, then a broken neck won't hurt so much.

 

[Idok]

a heavier hull will definatly have a smoother ride in choppy water




many yamahas are partially constructed with SMC (sheet molded compound)

 

[Waternut]

78 kilos bare?!?! That's 172 lbs for others and that's exceptionally heavy for a bare stand up hull and 10mm thick (.400 inches) is ridiculously thick! I'm far from a hull designer but I can't see how that kind of strength in a stand up would be necessary for anything the human body could physically handle.

Aircraft parts are much lighter and always much more expensive. In the aircraft world, weight is everything so parts are designed with low margins of safety. With those low margins of safety comes very strict inspection criteria. Boats and jetskis barely get looked at once a year. Boats aren't always fully designed either because on such a small scale it's cheaper to make it stronger than it is to spend a couple years analyzing stresses.

Under normal conditions your average aircraft, with the exception of fighters and acrobatic planes, will rarely see harsh conditions. Your average stand up hull, especially in the surf, gets beat up and slammed all day long.

 

[Foximus]

Guess you were looking at a SXR. They have a skeletal structure primarily of mysterious green glue.


If you ask me, the Green Goo is what makes them run.

 

[jimjet]

Thanks for the info guys. I think you have all given me a little insight as to the weight problem.
I know Polyester resin in Thailand is about 3$ a kilo and Epoxy is about 20$ a kilo so I can see why people use Polyester.
The problem is that the resin to fibre ratio with Polyester is 2 to 1 and epoxy is 1 to 1. Also epoxy is so much stronger that you only need to use half as much resin and half as much fibre which cuts the cost down to around the same as using polyester. You end up with a stronger lighter product that will outlast polyester.
Have you ever seen an F1 car made from Polyester resin? er no.
Nickwads ski is so light it is a bit scary but the idea was to make it as light as possible and see what breaks. He can always strengthen that area later but if he made it mega strong and nothing broke then he would never know how light he could have made it.
So far so good with. No gel coat either which has saved weight and nothing to crack. He seems to be trying to break it though.

 

[Waternut]

The problem is that the resin to fibre ratio with Polyester is 2 to 1 and epoxy is 1 to 1. Also epoxy is so much stronger that you only need to use half as much resin and half as much fibre which cuts the cost down to around the same as using polyester. You end up with a stronger lighter product that will outlast polyester.
Have you ever seen an F1 car made from Polyester resin? er no.

Ummm... I don't mean to be rude but almost all of that first paragraph is wrong. Yes, epoxy resin is stronger than polyester resin but polyester and vinylester resins are not inferior in all regards. However, fiber to resin ratio should be the same no matter what resin it is because you need complete wet out of the fabric and the weight difference between polyester and epoxy is barely even a factor. The fabric used plays a huge factor in how much resin is required though.

Also you'll never see an F1 car get made out of any wet lay up materials. They use prepregs fabrics and cook them in autoclaves because that is the lightest and strongest combination you can get. Granted the prepregs are a very high quality epoxy resin though. Most high performance sailing yachts are made from vinylester resin because you can't fit a huge boat like that in an autoclave.

 

[jimjet]

I was looking in a german catalogue and they give resin to fibre ratios.
E glass was 87 %, carbon 127% and chopped strand with polyester was 200%.
My original point was "why do people still use chopped strand and polyester"?
Is there an advantage to polyester that I don't know about?
With these figures polyester is at least 1/3rd heavier than an equivalent epoxy layup and it will not be as strong.
Is there a disadvantage to using epoxy other than cost?
I realise F1 cars don't use a wet lay up but my point was that they don't use polyester resins either.
I hope there are large safety margins in aircraft but I take your point that jetskis will be abused in all sorts of ways that aircraft would not be allowed to do.

 

[Waternut]

Doesn't matter if you use epoxy or polyester, the resin to fiber ratios are there as a guide in order to help with a complete wet out of the fabric. Different fabric require different amount of resin and chopped mat requires the most. Carbon is also lighter than E glass so therefore it's resin content will have to be a greater percentage of the total weight of the fiber and resin. I don't remember exactly but I want to say that the fiberglass is usually 40% fabric and 60% resin where carbon is 30% fabric and 70% resin. The carbon will still be lighter overall and the chopped mat will be the thickest because it absorbed so much resin.

I won't argue with the chopped mat though... It's crap and has no business in structural applications. The composite industry gets more and more complicated the more you learn so you can't look at just a couple numbers and get the full picture. You have to look at everything including elongation, strength, stiffness, etc.

 

[Foximus]

Doesn't matter if you use epoxy or polyester, the resin to fiber ratios are there as a guide in order to help with a complete wet out of the fabric. Different fabric require different amount of resin and chopped mat requires the most. Carbon is also lighter than E glass so therefore it's resin content will have to be a greater percentage of the total weight of the fiber and resin. I don't remember exactly but I want to say that the fiberglass is usually 40% fabric and 60% resin where carbon is 30% fabric and 70% resin. The carbon will still be lighter overall and the chopped mat will be the thickest because it absorbed so much resin.

I won't argue with the chopped mat though... It's crap and has no business in structural applications. The composite industry gets more and more complicated the more you learn so you can't look at just a couple numbers and get the full picture. You have to look at everything including elongation, strength, stiffness, etc.

Hrmm... I think i smell some poopy fingers..

Have you been pulling these numbers out of somewhere you shouldn't?

 

[Waternut]

Whatever dude... I did specifically state that I don't remember exactly and I'm not going to look up the information in a structural repair manual if no one is going to use it anyway. Not that it would do you any good anyway unless you are going to use the specific resin and fabric called out because it changes based on what you're using. The statement you are calling me out on is generic information to help the original poster understand what he's reading in the catalog and not step by step procedures for laying up something.

 

[Foximus]

Okie dokie... if you say so.... dude.


Though just for future use.... carbon and e glass weigh the same...


8oz is 8oz..... :bump:

 

[Waternut]

Okie dokie... if you say so.... dude.


Though just for future use.... carbon and e glass weigh the same...


8oz is 8oz..... :bump:

Sounds like you were so quick to make random accusations that you missed the entire point... 8oz is 8oz in weight but 8oz of carbon is MORE fabric (greater square footage) than 8oz of fiberglass and therefore requires MORE resin.

 

[Foximus]

:ugh2:

 

[jimjet]

Fibre is measured by the square metre in Europe and 100 grm e glass means 1 square metre weighs 100 grms.
100 grm carbon means that 1 square metre also weighs 100 grms.
Could be thicker though?? I must check that out.
What is it in the states? I mean what is 8oz? Is that 1 square yard?

 

[WaveDemon]

What is it in the states? I mean what is 8oz? Is that 1 square yard?

yes, cloth is measured in square yards

mat is measured in square feet.

 

[Foximus]

Yea, dont mind JFW... hes just bein playful...:highhorse:
:lmao:

 

[WaveDemon]

Sounds like you were so quick to make random accusations that you missed the entire point... 8oz is 8oz in weight but 8oz of carbon is MORE fabric (greater square footage) than 8oz of fiberglass and therefore requires MORE resin.

8 oz of glass is for 1 square yard

8 oz of carbon is for 1 square yard

the weight is the same and the size is the same. The thickness will be different and the strength is different.

 

[jimjet]

Thanks for all your help guys.
I just downloaded the latest R&G catalogue. This the holy bible for all laminating guys.
I must admit to being wrong on one point as I said chopped strand was 200% resin whereas it is actually 300 % resin to fibre weight.
I was interested in the figures so I will list a few.
291 grm eglass requires 233 grms resin
300 grm chopped strand requires 900 grms of resin. wow
110 aramid 155 grms resin
65 grm carbon requires 78 grms resin
This is all based on a square metre.
I tend to weigh the glass and mix the resin accordingly.
Am I being boring, sorry.
Check out the fantastic R&G catalogue though it is full of everything you could ever want and it gives you fibre specs and stuff.
No I don't work for them. ha ha

 

[kevbo]

I just found some information on material weights. E-glass has a specific gravity of 2.55. Carbon fiber averages 1.75. Epoxy resin around 1.25. Resin content is normally 30-32% by weight for autoclave cures and 50% for open air cures. With carbon costing 10X glass the weight savings can be $100 per pound depending on thickness. A carbon laminate can be a little thinner than glass reducing both weight and cost by maybe 10%. This is where the resin becomes the weak link because of the flatness of our hulls and is also where the cost start rising again if more strength is needed. Be aware of what you are buying in an aftermarket hull. Its very easy for the builder to substitute cheap material and sell it for top dollar.

 

[jimjet]

I checked to make sure I wasn't talking bollocks.
I took 80 grms of fibre and it took 70 grms of resin to wet it out fully.
This was done on a sheet of plastic. So R&G claim an 87% resin to fibre lay up which is about what I obtained.
I helped Nickwad when he laid up his new hull and he only used high quality epoxy resin with eglass which is woven cloth. No chopped strand anywhere.
This is why he has been able to reduce the weight so much.
Epoxy resin is about 7 times the cost of polyester in Thailand but neither of us like polyester.
This was my original question but no one has really answered it.
Is there any advantage to using polyester over epoxy other than cost?
I rode Nicks ski today, it feels very powerful considering the engine but I guess the light weight contributes to this.